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The best way to answer this question might be to consider the consequences if the input impedance was low: with a low input impedance, (signifficant) current would start flowing, and the amplifier would draw energy from the signal sources. None of the typical signal sources is designed to deliver energy on its outputs (after all, this is where the amplifier itself comes in). It is certainly possible to think that some of these sources might be changed to deliver some energy, but this is not the case with present-time tuners, CD players, microphones, and so forth. Assuming that the energy supply was not the issue, just to ponder this theoretical scenario a little further, the fact that current would flow from the source to the amplifier would also make the signal more vulnerable to the characteristics of the cable that connects the two. The high impedance of an amplifier input draws no energy, thereby avoiding these issues. It is the amplifier's task to convert a very low energy, voltage-driven signal into an higher energy output signal (driving the speakers which themselves have a very low impedance). ---- The way I typically think about this is to consider connecting a load to a Thevenin equivalent circuit [1]. The voltage across the load is given by the voltage divider formula (Vload = Vsrc * Rload/(Rload+Rthevenin)). If there is a very low load impedance--this means the amplifier has a very low input impedance--most of the source voltage will drop over the Thevenin equivalent resistance. With a very high input impedance, however, the majority of the signal voltage will be transferred from the source to the load because in the above equation, if Rload >> Rthevenin, Vload is approximately equal to Vsrc. if an amplifier has low impedance input the f/b must be low impedance also which make it in practical to use. The hi impedance of a typical amplifier is because the input is one two diodes basically operating on it exponential curve. Making it virtual the same as the other diode. for a differential amplifier. Boltzmann constant will define the impedance of a single diode.

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โˆ™ 2009-05-20 18:27:26
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Q: Why do amplifiers have a high input impedance?
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Related questions

What are the importance of impedance matching in amplifiers?

Audio amplifiers use no impedance matching. We use bridging. That means low output impedance and high input impedance.


Why the internal resistance should be high in operational amplifiers?

High input impedance implements the operation of the external feedback circuitry (e.g. diff. amplifiers vs. comparators).


How you can eliminate loading effect in amplifiers?

By increasing the input impedance of amplifier.


Compared to bipolars FETs have-a high input impedance b low input impedance c same input impedance?

FET s have very high input impedance when compared with Bipolar transistors.


In a multistage amplifier input impedance is high and output impedance is low justify the statement?

no. input impedance is low & output impedance is high


What are typical values of input impedance and outoput impedance of ce amplifiers?

Output of the power amplifier is smaller 0.1 ohms and input of the loudspeaker is more than 4 ohms.


Why voltage amplifier have high input impedance and low output impedance?

Voltage amplifier have high input impedance and low output impedance to use impedance bridging. The load gets the high voltage which comes from the source output.


Why do amplifiers have a low output impedance?

To get all the audio voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is at least ten times higher then the output impedance.An input impedance is called also a load impedance or an external impedance.An output impedance is called also a source impedance or an internal impedance.


Comparison between BJT and FET?

bjt has low input impedance but fet has high input impedance


Which is better high input impedance or low input impedance and why?

That depends on the output impedance. In electronic we use voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance. An input impedance is called also a load impedance or an external impedance. An output impedance is called also a source impedance or an internal impedance.


Why is JFET used as an input device?

it has high input impedance


Why input impedance of jfet is high?

input is reverse biased


Why the input resistance of opamp is high?

Because op amp consist differential amplifier and they posses high input impedance so that op-amp also posses high input impedance.


What is high input impedance?

To get all the voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance.An input impedance is called also a load impedance or an external impedance.An output impedance is called also a source impedance or an internal impedance.


Why input impedance should be high for an amplifier?

To get all the voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance.An input impedance is called also a load impedance or an external impedance.An output impedance is called also a source impedance or an internal impedance.


Why a high input impedance?

A circuit (an amplifier) typically has a high input impedance so that it does not unduly load the input circuit. Any such load could cause distortion, and that is generally not desired.


What are the main differences between FET's and BJT's in terms of input impedance?

FET has very high input impedanceBJT has very low input impedance


Why input impedance of any amplifier should very high?

To get all the audio voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is at least ten times higher then the output impedance.An input impedance is called also a load impedance or an external impedance.An output impedance is called also a source impedance or an internal impedance.


Input impedance is high and output impedance is low justify the statement?

To get all the voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance.An input impedance is called also a load impedance or an external impedance.An output impedance is called also a source impedance or an internal impedance.


Does a Digital Voltmeter have a high or low input impedance?

Digital voltmeter has high input impedence.


Why emitter follower preferred to transforms as a impedance matching?

because it has high input impedance and low output impedance


Why do you need high input impedance?

High input impedance is used with devices that need to measure a voltage without affecting the thing being measured. If it has a lower impedance, it would load down the source and affect it.


Why is it always desirable to have high input impedence and low output impedance for any circuit design?

To get all the voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is more than ten times higher then the output impedance.An input impedance is called also a load impedance or an external impedance.An output impedance is called also a source impedance or an internal impedance.


Why input impedance of op amp is high?

High input impedance is essential for voltage bridging. No power transfer and no heavy load. Scroll down to related links and look at "Impedance bridging - Wikipedia".


Why input impedance of operational amplifier is infinite?

Input impedance (Zin) is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry. Infinite Input impedance is one of the Ideal Characteristics of the Op-Amp. With an assumption of Infinite Input impedance, there is no Loading on the preceeding stage to the Op-Amp (i.e. Supply.) or The Op-Amp under test does not draw any current from the I/p Supply to it's internal Circuitry.